1. Field of the Invention
This invention relates to an organic electroluminescent device and, more particularly, to a thin film device having a light-emitting layer of an organic compound which emits light upon electric field application.
2. Background Art
Conventional thin film electroluminescent (EL) devices generally comprise an inorganic material, such as a group II-VI compound semiconductor, e.g., ZnS, CaS or SrS, doped with Mn or a rare earth element (e.g., Eu, Ce, Th or Sm) as a luminescence center. EL devices made of these inorganic materials have such disadvantages as (1) necessity of alternating current drive (50 to 1000 Hz), (2) a high driving voltage (up to 200 V), (3) difficulty of full color light emission (particularly in blue), and (4) high cost of peripheral driving circuits.
To eliminate these disadvantages, EL devices using an organic thin film have recently been developed. In particular, in order to raise luminescence efficiency, the kind of an electrode has been optimized for improving efficiency in carrier injection from an electrode. Further, an organic electroluminescent device having a hole transport layer comprising an aromatic diamine and a luminescent layer comprising an aluminum complex of 8-hydroxyquinoline has been developed (Appl. Phys. Lett., vol. 51, p. 913 (1987)). Thus, organic EL devices have shown great improvements on luminescence efficiency over conventional ones comprising single crystals of anthracene, etc. to gain characteristics approaching the level meeting practical use.
In addition to the electroluminescent devices using the above-described low-molecular-weight materials, those using high-molecular-weight materials such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], and poly(3-alkylthiophene), and those using high-molecular-weight materials, such as polyvinylcarbazole, mixed with low-molecular light-emitting materials and electron transfer materials have also been developed.
Under these circumstances, the outstanding objects relating to organic electroluminescent devices include improvement in driving stability and reduction of driving voltage.
That is, it is a great problem for a light source, such as a backlight of facsimiles, copiers and liquid crystal displays, that an organic electroluminescent device requires a high driving voltage and has low driving stability including heat resistance. This is especially undesirable for such display devices as full color flat panel displays.
Factors of driving instability of organic electroluminescent devices include reduction of luminescent brightness, voltage increase in constant current drive, and development of non-light-emitting parts (dark spots). While there are a number of causes of these instability factors, deterioration of the cathode material, particularly the interface at the light-emitting side of the cathode seems to be a chief cause. In an organic electroluminescent device a metal of low work function, such as a magnesium alloy or calcium, is usually used as a cathode material in order to facilitate electron injection from the cathode into the organic layer. Such a metal is susceptible to oxidation with moisture in air, which is a large factor of driving instability. An electrode made of a low work function metal, while effective in lowering the driving voltage, needs improvements to overcome the above-mentioned instability.
On the other hand, a cathode comprising aluminum containing 0.01 to 0.1 part by weight of metallic lithium has been proposed (an unexamined published Japanese patent application 5-121172). Formation of this cathode requires strict control on the metallic lithium content. However, it is technically difficult to form a cathode layer of an aluminum-lithium alloy having a desired composition by binary vacuum deposition using aluminum and metallic lithium as independent deposition sources. It is conceivable to form a cathode by electron beam deposition or sputtering using a previously prepared pellet or target of an aluminum-lithium alloy having a desired composition. This method, however, involves a practical problem that the composition of the aluminum-lithium alloy deposition source will vary as film formation is repeated due to the differences between lithium and aluminum in vapor pressure or sputtering efficiency. Besides, use of lithium is disadvantageous in that metallic lithium atoms are apt to diffuse into the adjoining organic layer, causing extinction of luminescence and that lithium atoms are so sensitive to moisture that a device having a lithium-containing cathode strictly demands high accuracy of sealing.
A cathode comprising an aluminum alloy containing 6 mol % or more of lithium is also disclosed (an unexamined published Japanese patent application 4-212287). With this cathode, too, a device requires a strict protective film on account of the above-mentioned instability of metallic lithium atoms and cannot get rid of the instability due to diffusion of lithium atoms.
A cathode made of aluminum metal mixed with an alkali metal fluoride has been reported (Appl. Phys. Lett., vol. 73, p. 1185 (1998)), which gives no considerations for device stability.
A two-layered cathode having Li2O and Al in independent layers has been proposed (IEEE Transactions on Electron Devices, vol. 44, No. 8, pp. 1245-1248 (1997)). In this technique, however, because a very thin film of 0.5 to 1.5 nm is used as a cathode interfacial layer, it appears that the film may fail to completely cover the organic layer, and reproducibility seems insufficient. Additionally Li2O has poor adhesion to an organic layer as compared with Al and may cause formation of dark spots.
Thus, cathode materials heretofore proposed for organic electroluminescent devices include aluminum metal alloyed with lithium or mixed with a lithium compound as stated, but none of them is effective in improving driving stability and reducing a driving voltage, or they involve a practical problem in the process for production.
An object of the present invention is to settle the above-described problems and to provide an organic electroluminescent device which can be driven at a low voltage with a high luminescence efficiency, maintains stable luminescence characteristics for an extended period of time, and exhibits excellent resistance to heat and weather and a process for easily producing such an organic electroluminescent device without requiring strict condition control.
The organic electroluminescent device according to the present invention comprises a substrate having a luminescent layer provided between an anode and a cathode, which is characterized in that the cathode comprises a metallic material, an alkali metal, and an oxygen atom.
The present inventors have conducted extensive studies, seeking for an organic electroluminescent device that exhibits excellent resistance to heat and weather, emits light with high brightness at a low voltage, retains stable luminescence characteristics in driving, and can be produced under a broad range of process conditions. As a result, they have found that the above objects are accomplished by making the cathode of a metallic material containing an alkali metal and an oxygen atom, and completed the present invention.
In the present invention, incorporation of an alkali metal into a cathode makes it possible to lower the work function of the cathode thereby to reduce the energy barrier of the cathode interface for electron injection. As a result, there is produced an effect on lowering the driving voltage of the device. Further, existence of oxygen atoms suppresses diffusion of alkali metal atoms such as lithium atoms into an adjacent layer. Because it already contains oxygen, the cathode is chemically stabilized against the external environment, such as an oxidizing environment. In other words, part of oxygen introduced into the cathode is bonded to the metallic material and/or the alkali metal to form an oxide of the metallic material, in the presence of which the alkali metal atoms are suppressed from diffusing. Existence of carbon atoms enhances affinity to an organic layer to improve the adhesion of the cathode to the organic layer. As a result, there is obtained a device that emits light with high brightness and high efficiency at a low voltage, exhibits stability in driving at a high current density, and hardly undergoes deterioration in storage.
The terminology xe2x80x9cmetallic materialxe2x80x9d as used herein is intended to mean a metal except alkali metals or an alloy thereof, which constitutes the main component of a cathode.
The cathode of the invention generally has a metallic material content of 50 to 95 at. %, preferably 60 to 90 at. %; an alkali metal content of 0.1 to 20 at. %, preferably 0.2 to 10 at. %; and an oxygen atom content of 1 to 40 at. %, preferably 3 to 30 at. %. The cathode of the invention is allowed to have a carbon atom content of not more than 30 at. %, preferably not more than 10 at. %, that is, carbon atoms may be incorporated together with some forms of an alkali metal. The metallic material includes one or two or more members selected from the group consisting of aluminum, indium, magnesium, calcium, zinc, vanadium, chromium, tin, and copper, with aluminum or an aluminum alloy being preferred.
The alkali metal which is incorporated into the cathode of the invention usually includes lithium, sodium, potassium, cesium, and mixtures thereof, with lithium and sodium being preferred.
It is preferred that a metal layer containing no alkali metal be provided on the cathode opposite to the luminescent layer.
The organic electroluminescent device according to the invention can easily be produced with no strict restrictions on process conditions by the process of the invention which has the step of forming a cathode according to any one of the following methods (1) to (4):
(1) The cathode is formed by reactive vacuum deposition in an oxidizing atmosphere using a metallic material and an alkali metal as deposition sources. In this method, it is preferable to use an alkali metal nitride as an alkali metal deposition source.
(2) The cathode is formed by vacuum deposition using a metallic material and an alkali metal oxide as deposition sources.
(3) The cathode is formed by reactive sputtering in an oxidizing atmosphere using an alloy composition comprising a metallic material and an alkali metal as a target.
(4) The cathode is formed by sputtering using a composition comprising a metallic material, an alkali metal and oxygen as a target.
By following any one of the methods (1) to (4), a satisfactory cathode can be formed without strict process control because the range of the alkali metal content that promises a device satisfactory luminescence characteristics is broad.
The organic electroluminescent device of the invention can also be produced with ease by the process of the invention with no strict restrictions imposed on process conditions, which process has the step of forming a cathode by simultaneous vacuum deposition of a metallic material and an organic compound containing an alkali metal as deposition sources.
In this process, the organic compound containing an alkali metal partly reacts with the metallic material, such as aluminum, on the substrate upon evaporation in vacuo, whereby the alkali metal is released therefrom and incorporated into the cathode. As a result of the reaction of the organic compound, carbon atoms as well as oxygen atoms are also incorporated into the cathode and exert inhibitory effect on alkali metal diffusion, playing a role in improving stability of the device. In this simultaneous vacuum deposition, because the reaction on the substrate (release of the alkali metal) is a rate-determining step for the amount of the alkali metal atoms to be taken into the metallic material, an alkali metal content that is optimum for a device can be provided from a broad range of the evaporated amount of the alkali metal organic compound. Accordingly, a desired alkali metal content can be obtained only if the amount of the evaporated organic compound is controlled. Thus, compared with conventional methods of forming an alloy cathode, the method of the invention enjoys markedly broadened freedom of process conditions such as evaporation time, furnishing considerable process merits, and is effectively applicable to large-volume production.